1. Field of the Invention
The present invention relates to a thermistor device and a method for manufacturing the same.
2. Description of the Related Art
Thermistor devices, which have been used as a temperature sensor in household appliances, housing equipments, automotive equipments and the like, are generally composed of a thermistor element unit and a terminal plate. The thermistor element unit is assembled with a thermistor element disposed between two dumets, and the assembly is sealed by a glass body. Moreover, a lead conductor attached to the dumet is electrically connected to a terminal plate.
Thermistor devices of this type are basically required to have a tight joint between the lead conductor and the terminal plate. This is because if the mechanical joint strength between the lead conductor and the terminal plate is insufficient, mechanical or thermal stress externally applied to the thermistor device tends to cause a gap due to separation or breakage at the joint between the lead conductor and the terminal plate, leading to the separation problem of the lead conductor. If such a gap or the separation problem of the lead conductor occurs, the thermistor element unit becomes electrically open from the terminal plate, impairing the function and reliability as a temperature sensor.
Concerning the joint between the lead conductor and the terminal plate in the prior art, for example, Japanese Patent No. 3039277 discloses a thermistor device in which the lead conductor and the terminal plate are joined together by soldering. On the other hand, Japanese Patent No. 2601046 discloses a thermistor device in which the lead conductor and the terminal plate are joined together by caulking and then electric resistance welding.
However, because soldering disclosed in Japanese Patent No. 3039277 cannot provide sufficient mechanical joint strength between the lead conductor and the terminal plate, mechanical or thermal stress externally applied thereto tends to cause a gap due to separation or breakage at the joint therebetween, leading to the separation problem of the lead conductor.
Moreover, because soldering in the process of manufacturing precision electronics devices of this type is generally performed by a reflow method using a furnace, the thermistor element may react to heating at the time of reflow soldering, deteriorating in thermistor characteristics.
Furthermore, lead contained in solder is now known to cause an environmental issue, which requires us to restrain ourselves from using lead solder or to meet use restrictions such as switching over to lead-free solder.
On the other hand, because electric resistance welding disclosed in Japanese Patent No. 2601046 requires the lead conductor to be pressed against the terminal plate, the stress upon assembly may put a burden on the lead conductor and the terminal plate, deteriorating the reliability. In addition, pressing the lead conductor against the terminal plate tends to cause a displacement when these components come into contact with each other upon assembly, impairing the function and reliability as a temperature sensor.